The present invention relates to Raman resonators.
Raman amplifiers and resonators are known in the field of optical communications. These devices rely on the Raman effect. When light is transmitted through matter, part of the light is scattered in random directions. A small part of the scattered light has frequencies removed from the frequency of the incident beam by quantities equal to vibration frequencies of the material scattering system. This small part is called Raman scattering. If the initial beam is sufficiently intense and monochromatic, a threshold can be reached beyond which light at the Raman frequencies may be amplified, generally exhibiting the characteristics of stimulated emission. This stimulated emission is commonly referred to as the stimulated Raman scattering.
One device employing the Raman effect is a cascaded Raman resonator (xe2x80x9cCRRxe2x80x9d). Generally, a CRR receives radiation from a source pump at a particular wavelength, xcexpump, and shifts the radiation through one or more steps to a desired output wavelength, xcexout, where xcexout is greater xcexpump. While various types of CRRs exist, one type currently being examined is a fiber-based CRR, which shifts the wavelength of the pump light in an optical fiber. Fiber-based CRRs are capable of providing higher power in a single mode fiber than single mode semiconductor diodes. To date, fiber-based CRRs have been used for remote pumping of Er-doped fiber amplifiers, and as pumps for Raman amplifiers.
In an optical fiber, the gain curve from the Raman effect is relatively broad, yet not particularly flat over a wide frequency range. To obtain a flat gain curve, a Raman amplifier may be pumped using several different wavelengths, each triggering the Raman effect. The gain profile of such a Raman amplifier is effectively the superposition of the gain of each of the individual pumps, in addition to the interaction between the pumps. Presently, these pumps have been realized by multiplexing a number of semiconductor laser diodes or CRRs together. Multiplexing schemes, however, add additional cost to the overall device and place wavelength and polarization limitations on the semiconductor diodes. The power required from each single wavelength device is modest when compared to the total power that a CRR is capable of producing. However, the total power in all of the wavelengths is comparable to that obtainable from a CRR. It has therefore been advantageous to turn the large amount of power available at a single wavelength of a CRR into power at multiple wavelengths.
One practical solution for making a multiple wavelength cascaded Raman resonator (xe2x80x9cMWCRRxe2x80x9d) has been to variably distribute power over the output wavelengths. This approach has been disadvantageous because the tolerances imposed by a system on the wavelength power ratio of a MWCRR are tighter than the possible manufacturing tolerances. Moreover, the specifications imposed by the system also depend on the final assembly of the system. The performance of the system, consequently, may be enhanced by dynamically controlling the wavelength power ratio and, hence, the shape of the gain curve. As such, a need remains for the ability to control the wavelength power ratio of a MWCRR.
We have invented a method for controlling the relative wavelength power distribution in a Raman device, such as, for example, a MWCRR. In accordance with the present invention, an optical device employs at least one output coupler having a reflectivity which may be independently varied or tuned to compensate or achieve a desired power distribution. The reflectivity of the output coupler may be modified using various means, including, for example, applying a non-uniform stress, heat or a voltage/current.
In one example of the present invention, a Raman device, such as, for example, a MWCRR, comprises at least one set of optical gratings coupled with at least a first and a second output coupler for controlling the relative wavelength power distribution. Here, each output coupler has a reflectivity which varies in response to the application of a non-uniform stress, heat or a voltage/current.